President Joe Biden has pledged United States support for Ukraine should Russian-backed separatists backed by the Russian Army and the Armed Forces of Belarus attack its eastern provinces.

Biden made the extraordinary commitment of U.S. military power to Ukrainian president Volodymyr Zelensky in a 50 minute-long phone call Saturday. The Biden-Zelensky conversation, their first, comes amid a dangerous build-up of Russian and Belarussian military forces in the Donbas region, which Western military analysts say has the hallmarks of an impending invasion.

Biden offered Zelensky the United States' "unwavering support" as Ukraine continues to defy Russia's blatant attempt to militarily annex the Donetsk and Luhansk regions in eastern Ukraine using pro-Russia separatist groups as proxies.

"President Biden affirmed the United States' unwavering support for Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression in the Donbas and Crimea," said a statement from the White House.

"We stand shoulder to shoulder when it comes to preservation of our democracies," tweeted Zelenskiy after the call.

Zelensky said he and Biden "discussed the situation in Donbas in detail. President Biden assured me that Ukraine would never be left alone against Russia's aggression."

The Biden-Zelensky conversation came after the U.S. and NATO voiced alarm over Russian troop movements near the eastern Donbas region, where Ukrainian troops are holding the line against Russian-backed Ukrainian separatists.

Biden also expressed his wish to visit Ukraine again, as he had as vice president, according to Zelenskiy's office.

On Thursday, U.S. Secretary of Defense Lloyd Austin spoke with his Ukrainian counterpart, Andrii Taran, and "condemned recent escalations of Russian aggressive and provocative actions in eastern Ukraine."

Russia responded the next day by saying the situation along the contact line in eastern Ukraine between Ukrainian government forces and Russian-backed separatist forces was quite frightening and that multiple provocations by the Ukrainian Army were taking place in the region.

It warned any deployment of NATO troops to Ukraine would lead to further tensions and force it to take extra measures to ensure its own security.

Western military analysts continue to warn of a war between Ukraine and Russia as they cite the unceasing build-up of Russian and Belarussian forces in areas controlled by pro-Russia separatists.

Ukraine has reacted to Russia's provocations by calling up its military reserves. Gen. Ruslan Khomchak, Commander-in-Chief of the Armed Forces of Ukraine, confirmed Russia is reinforcing its forces near Ukraine's borders in a threat to the country's security.

Khomchak accused pro-Russian separatists of systematically violating the July 2020 ceasefire in eastern Ukraine. Ukraine said the onflict against Russia and its Ukrainian allies has killed 14,000 people since 2014.